The subject invention relates generally to a variable capacity pump and, more particularly, to a variable capacity vane pump for delivering a constant flow output under variable pressure conditions.
Many industrial and automotive devices require a continuous supply of compressible fluid such as oil and fuel to operate. In order to obtain a given fluid output, a pump may be driven at a constant speed by means of an electric motor or, as more commonly found in automobiles, by utilizing the engine rotation to drive a pump shaft via a belt connection between a driving pulley (connected to the crankshaft of the engine) and a driven pulley. However, it is often desirable to maintain a constant fluid output irrespective of the engine speed. To meet this need, the following two types of pumps are commonly used:
1. A variable-capacity pump capable of delivering a sufficient fluid output even when the engine operates at a minimum speed. When the engine speed is increased, the capacity of the pump is proportionally reduced to keep the fluid output at a substantially constant value;
2. A constant-capacity pump designed for delivering the specified fluid output when the engine operates at a minimum speed. When the engine speed is increased, an increasing fraction of the pump output is diverted and returned to the reservoir (or the suction port of the pump) to maintain the fluid output at a constant value.
Variable capacity pumps are favored in that they offer a significant improvement in energy efficiency and can respond to changes in operating conditions more quickly than constant-capacity pumps. For example, automatic and continuously variable transmissions require oil pressures approaching 1200 psi. If a constant-capacity pump is used in this application, power consumption increases dramatically at higher engine speeds, such as those experienced under normal highway driving conditions, because the flow amount is directly proportional to engine speed. A pressure compensated pump also suffers from the problem of long response times when a clutch or hydraulic device is actuated.
U.S. Pat. No. 3,381,622 discloses a variable output roller pump with a constant output pressure. The pump comprises a mounting plate, a cavity body mounted to the mounting plate, a cam ring enclosed within the cavity body, and a rotor mounted about a fixed axis within the cam ring. The rotor includes a number of radial slots for retaining rollers. The mounting plate includes fluid inlet and outlet ports aligned with the root circle of the roller slots for respectively delivering and removing fluid to and from each slot as the rotor rotates. The pump also includes a leaf spring and a pressure conduit coupled between the cam ring and the leaf spring for reducing the eccentricity of the cam ring (and hence the output pressure) as the output pressure increases.
U.S. Pat. No. 3,642,388 discloses a variable output roller pump with a continuously variable output flow. The pump comprises a housing in which a rotor is rotatably mounted about a fixed axis within a surrounding cam ring. The rotor has a series of radial angularly spaced notches in which rollers are slidably mounted. The cam ring is rotatably coupled to a roller at one end and to a hydraulically operated piston at the opposite end for urging the cam ring between a maximum and minimum pump flow position in response to changes in hydraulic fluid pressure acting on the piston.
U.S. Pat. No. 4,679,995 proposes a variable capacity rotary pump similar to U.S. Pat. No. 3,381,622, except that the cam ring pivots about a roller at one end and is urged into a position of maximum fluid output by a spring seated at the opposite end. At the same time, a portion of pressurized fluid output exerts a force to counteract the spring force so as to automatically reduce the flow output of the pump when the output pressure increases.
In each prior art example, differences in the fluid pressures of the fluid chamber entering the outlet port and the fluid chamber exiting the outlet port can cause undesirable variations in the output pressure of the pump. Accordingly, there remains a need for a variable capacity pump that provides a constant fluid flow under variable output pressures.
The present invention provides an oil pump construction with its capacity variable in order to keep the pump flow constant and independent of engine speed or line pressure.
The variable capacity pump comprises a housing, a rotatable rotor within the housing. The rotor includes radial slots to accommodate slidable vanes or rotor blades, wherein the vanes are urged outwards by centrifugal force into contact with the inner surface of the surrounding cam ring. The cam ring is surrounded on one end by a pressure chamber and on the other end by a seated spring.
A venturi tube is preferably employed to obtain the differential pressure necessary to measure the flow being delivered by the pump and to give a feedback signal to a hydraulic control valve to adjust the pump capacity.
The control valve may be a spool valve. The spool valve is biased to a rest position and operates to connect the pressure chamber to either a discharge port or a high pressure output line whenever the pressure differential of the main output across a venturi tube exceeds a predetermined value. By controlling the pressure distributed to the pressure chamber, the position of the cam ring with respect to the rotor may be changed to automatically vary the displacement of the pump.
In another embodiment the control valve is eliminated, and a pivot pin defines two control volumes acting on either side of the cam ring. Differential fluid pressure acting on these control volumes controls the cam ring position or eccentricity directly.